The 40-Bushel Reason to Wait on Fall Nitrogen

Combines are starting to roll, fields are opening up, and the temptation to knock out fall fertilizer applications will build as harvest picks up steam. But Michael Castellano, Iowa State University Extension agronomist, has a valuable message for farmers in a hurry: Pump the brakes.

Moving too fast on fall anhydrous and manure applications could turn a good nutrient investment into an expensive mistake — one that could cost you up to 40 bushels per acre of corn yield next season.

Wait For The Right Soil Temperature

Castellano’s guidance to farmers: Wait until soil temperatures at a 4-inch depth reach 50 degrees Fahrenheit and are trending downward.

That downward trend is just as important as the temperature reading itself. A brief cold snap that drops soils to 50 degrees before they quickly warm back up doesn’t count. Watch the forecast and make sure soils are on a cooling trend.

“Let the soil — not the harvest clock — determine when fall applications begin,” Castellano says.

For example, in central Iowa, Nov. 10 typically marks the benchmark for reaching that 50-degree threshold. But this year could be different.

El Niño Could Delay Fall Fertilizer Application Timing

Strong El Niño conditions in the Pacific are setting up what could be an extended warm fall across the Corn Belt — and that means farmers in the Midwest may need to wait longer than usual to safely apply nitrogen.

“Last time we had a strong El Niño, the fall temperatures were significantly above average. They were 3 degrees above average for September through October, and 10 degrees above average in December,” Castellano recalls.

This year, he expects the typical mid-November timing could be pushed back by as much as 10 to 15 days. And for farmers with fields that caught some rain recently, the wait could stretch out even longer.

“Wet soils take longer to cool down than dry soils, so we really could be set up for some extended warm soils this fall,” he told AgriTalk host Chip Flory on Monday.

What Happens When You Apply Too Early?

When anhydrous ammonia or manure goes on warm fields, soil microbes get to work converting ammonia-based nitrogen into nitrate — a form of nitrogen that’s highly vulnerable to loss via water or air.

“The potential for loss to the air is particularly high because microbes can then denitrify that nitrate to nitrogen gasses that go into the atmosphere,” Castellano explains.

That’s not just lost nitrogen. It’s lost money and lost yield potential.

Iowa State University research on manure applications drives the point home. Apply too early, and Castellano says the hauling costs can actually exceed the nutrient value you’re putting down.

“Our research also shows with the same rate of manure, if it’s applied before the soil’s 50 and falling, you can lose as much as 40 bushels an acre of corn,” he says.

Nitrogen Inhibitors Can Protect Your Investment

Castellano understands that farmers’ reality doesn’t always cooperate with ideal application timing. Manure storage limits, custom applicator schedules, harvest pressures, and narrow weather windows can force your hand.

If you have to apply early, he says to be sure to protect your investment.

“You got to use an inhibitor,” Castellano notes. “We always recommend an inhibitor with anhydrous ammonia, and there are also inhibitors available for manure.”

With nitrogen prices still at elevated levels heading into fall, this could be a particularly good year for that kind of protection to pay dividends.

“We’re really set up for an ideal year for inhibitors to pay,” he says.

Cover Crops Reduce Risk Of Loss

Cover crops offer another layer of defense for farmers, especially for those operations that face early manure application deadlines.

“If you have to get it out there early, make sure you’ve got a cover crop growing,” Castellano advises.

Applying manure over a growing cover crop can substantially reduce both the agronomic and environmental risks of an early application, he adds.

Hear all of Castellano’s recommendations by checking out the discussion he had with Chip Flory on AgriTalk, available at the link below: